1. Field of Invention
The present invention relates to the field of electrochemical generators, and more particularly to that of metal-air storage batteries and systems.
Specifically, it relates to secondary generators with a zinc anode and is intended to obtain a high level of cyclability of the zinc electrode.
The zinc electrode is well-known to the person skilled in the art for its high level of performance. It may furthermore be used in various secondary electrochemical systems: alkaline air-zinc, nickel-zinc and silver-zinc generators, bromine-zinc and chlorine-zinc generators with saline electrolytes.
2. Description of Related Art
Zinc is an attractive anodic active material, having a strongly negative redox potential of −1.25 V/NHE for the pair Zn/Zn(OH)2. The zinc electrode offers a theoretical gravimetric specific capacity of 820 Ah/kg. It accordingly makes it possible, for example, to achieve theoretical gravimetric specific energies of 334 Wh/kg for the nickel-zinc pair (NiZn), and of 1,320 Wh/kg for zinc-oxygen pair. For an NiZn storage battery, the practical gravimetric specific energy may be between approximately 50 and 100 Wh/kg, the voltage furthermore being 1.65 volts, instead of the 1.2 volts for other alkaline systems.
Further advantages of zinc which should be emphasised are, on the one hand, its non-toxicity to the environment (production, use, disposal), and, on the other hand, its low cost, which is very much less than that of other anodic materials for alkaline storage batteries (cadmium and metal hydrides), or lithium storage batteries.
However, the industrial development of rechargeable systems using a zinc electrode has encountered a major stumbling block, namely the electrode's inadequate cycle life.
The reactions which occur at the anode are as follows in an alkaline storage battery:

Recharging of the zinc electrode from the oxides and hydroxides thereof and from the zincates in fact generally gives rise to the formation of deposits with a structure which is modified relative to the original form, said deposits often being described as dendritic, spongy or powdery. This phenomenon moreover occurs over a very wide range of current densities.
Successive recharges thus rapidly result in the chaotic growth or outgrowth of zinc through the separators and in short-circuiting with the electrodes of the opposite polarity.
As for the powdery or spongy deposits, these prevent the reconstitution of electrodes capable of satisfactory or extended operation due to inadequate adhesion of the active material.
Furthermore, reduction of the zinc oxides, hydroxides and zincates to zinc at the anode during recharging is also characterised by morphological changes in the electrode itself. Depending on the mode of operation of the storage batteries, various kinds of changes in form of the anode are observed as a result of non-uniform redistribution of the zinc during the formation thereof. This may in particular result in a troublesome densification of the anodic active mass at the surface of the electrode, most often in the central zone thereof. At the same time, electrode porosity is reduced, which contributes to an acceleration in the preferential formation of zinc on its surface.
These major shortcomings, which reduce the achievable number of cycles to a few dozen (a level which is inadequate to ensure economic viability for a secondary system), have given rise to a large number of studies devoted to improving zinc deposition characteristics during recharging with the aim of increasing the number of charge-discharge cycles which the generator can accept.
Various very different approaches have been investigated with the objective of attempting to minimise or to delay as long as possible these zinc formation defects, the following of which may in particular be mentioned:                “mechanical” methods intended to reduce the chaotic formation or outgrowth of zinc, or to avoid powdery deposits: circulation of the electrolyte and/or of the zinc electrode in dispersed form; vibration imparted to the electrodes; use of separators which are resistant to perforation by dendrites, frequently in multiple layers, and even of ion-exchange membranes, in order to prevent the migration of zincates;        “electric” methods intended to improve the conditions under which the zinc deposit is formed: control of charging parameters (intensity, voltage etc.); use of pulsed current, including current inversion, in an attempt to dissolve the dendrites while they are forming;        “chemical” and “electrochemical” methods: use of additives incorporated into the electrolyte (fluoride, carbonate, etc.) and/or into the anodic active material (calcium, barium etc.) and dilution of the electrolyte, in particular in order to limit the solubility of the zincates and to form zinc oxide and insoluble compounds of zinc.        
These various methods may be implemented in isolation or in combination.
In any event, they have only limited positive effects which have proved inadequate to impart economic viability to secondary generators with a zinc anode and in particular to the nevertheless theoretically very attractive pair, NiZn; they barely make it possible to achieve or exceed around a hundred cycles performed with a significant depth of discharge.
Moreover, some of these methods have disadvantageous negative effects, such as:                increase in the internal resistance of the storage battery (due to certain additives or to electrolyte dilution),        reduction in the life of the nickel cathode (due to the use of certain additives),        mechanical complexity of operation (for circulating systems),        increases in the volume and mass of the system (impairment of specific performance parameters in terms of gravimetric and volumetric specific energies),        increased costs (losing the potential economic advantage).        
A major innovation was provided and described by the description of French patent application 99 00859, the developed technology making it possible to achieve several hundred cycles over a wide range of operating conditions and down to very deep depths of discharge thanks to the implementation of means intended to increase the efficiency of use of the active material by improving the percolation of charges within it.
The present invention is based on the observation that insufficient drainage of charges within the active material promotes the formation of the zinc deposit during recharging at sites which represent only a limited percentage of the entire active mass. This zinc growth, a phenomenon which most frequently gives rise to a chaotic deposit which may result in outgrowths through the separators or in densification of the deposit, accordingly proceeds from sites with a limited total surface area relative to the overall projected surface area of the anodic material. The technology described in the above-mentioned document shows that this mechanism may be greatly reduced if, by increasing the number of deposit formation sites, the same total quantity of zinc is deposited over a much larger surface area throughout the volume of the electrode.
According to a preferred embodiment, this technology results in the use, within the zinc anode, of two or three levels of electrical collection:                a main collector network: an electrode support of the “metal foam” type (reticulate honeycomb structure),        a secondary conductor network: a dispersion of conductive, chemically inert, ceramic particles in the storage battery,        a possible complementary tertiary conductor network: a dispersion of bismuth in the anodic active mass.        
An “antipolar mass”, which may consist of nickel hydroxide when producing nickel-zinc storage batteries, may also be introduced into the zinc anode and makes a significant contribution to the level of performance achieved.
The aim of the present invention is to improve the cyclability of the zinc electrode by prior treatment of the conductive ceramic, before the addition thereof to the active mass of the zinc electrode, the purpose of the treatment being to impart to said ceramic powder a second function of retaining the zincates formed on discharge of the zinc anode.
In the “Journal of the Electrochemical Society”, vol. 145, no. 4, page 1211, 1988, C. F. Windisch and al. describe the changes in polished discs of titanium nitride (TiN) which are immersed for 136 days in a concentrated solution of potassium hydroxide.
Compounds identified as slightly crystallised potassium titanates form on the surface of the material.
As emphasised by J. Lehto in U.S. Pat. No. 6,106,799, it is not always straightforward to distinguish between titanates and hydrated titanium oxides, since hydrated titanium oxides may be considered to be amorphous or semi-crystalline forms of titanates.
This same author points out that titanates and hydrated titanium oxides have ion-exchange properties, which are utilised for treatment of effluents containing radioactive ions.
Furthermore, titanium nitrides, in particular in powder form, are not inert with regard to atmospheric oxygen, even at ambient temperature.
Uncrystallised or slightly crystallised titanium oxynitrides and titanium oxides, which may be detected by XPS analysis, form on the surface of the grains.
RX analysis of TiN powders does not necessarily reveal the presence of titanium oxide, but a modification of the nitride lattice parameter corresponding to compounds which are sub-stoichiometric in nitrogen may be observed.
Commercial TiN powders are generally produced by nitriding titanium. They always exhibit nitrogen deficits of 0.5 to 2% relative to the stoichiometric quantity, which is 22.62% by weight. This deficit may be even larger when the powders are prepared by methods such as nitriding of titanium dioxide with ammonia, or synthesis by a self-propagating thermal reaction from titanium oxide, titanium halides, etc.
Titanium oxynitrides are of the general chemical formula TiNxOy, with x and y varying between 0.01 and 0.99.
At low oxygen contents, the (face-centred cubic) crystalline structure and the corresponding parameters of titanium oxynitrides are virtually identical to those of the nitride, which makes them difficult to identify by RX analysis. Titanium oxynitrides are black in colour, and if TiN is golden in colour, progressive oxidation of the TiN results in a colour change starting from bronze to brown and then to black.
This characteristic has been exploited for the preparation of black pigments as replacements for carbon, iron oxide or manganese dioxide powders.
Various methods have been described in the literature for preparing titanium oxynitrides:                partial reduction of the titanium dioxide by ammonia,        partial oxidation of very finely divided TiN,        manufacture of TiN by plasma discharge from titanium tetrachloride.        
The oxidation temperature of TiN powders, the process being accompanied by an increase in weight, is closely related to the nature of the samples. Fine powders of a diameter of approximately 5 μm begin to oxidise at around 350° C., whereas coarser powders of 50 μm will begin to do so at around 500° C. (P. Lefort and al., Journal of Less Common Metals, no. 60, page 11, 1978).
Extended, high-temperature treatment of TiN powder results in the formation of rutile-type titanium oxide. On the basis of this knowledge, the authors of the present invention thus discovered that oxidation pretreatment of TiN powders, the use of which in a zinc electrode is described in French patent 99 00859, improved the cyclability of the electrode, due to increased reactivity of the ceramic powder with regard to the electrolyte, so giving rise to the formation of hydrated titanium oxides which may themselves change into partially crystalline titanates.
It is thus possible to impart to the ceramic powder, dispersed in the anodic mass, two functions which are essential to the proper functioning of the zinc electrode:                an electron-conductive function, which will contribute to obtaining a more uniform distribution of the formation of metallic zinc within the active mass during successive charging operations,        a retention function for the zincates arising from the oxidation of the zinc on discharge of the storage battery, thanks to the formation of adsorbent compounds created by the pretreatment performed on the TiN powder.        
As a result of this pretreatment, which is performed on TiN powders which are themselves obtained by various processes, it will be possible greatly to accelerate the formation of zincate binding sites brought about by surface modification of the TiN in accordance with a mechanism described by Windish and al. In this way, it will be possible, right from the first formation cycles of the zinc electrode, to produce uniform deposits of zinc thanks to said second function imparted to the ceramic powders. Producing the most uniform possible deposits of zinc within the anode right from the first recharge cycles is essential to obtaining a significant cycle life of generators with a zinc anode.